I just fixed a plastic part on an older car which was broken in half and also had cracks. I used clear epoxy glue from the dollar store (clean with acetone) and just held the piece together for at least 10 minutes and had my partner put duct tape on the back which I cut to size before hand. Then I slowly filled the voids with car putty and sanded in between. You have to let the piece sit for a day or 2 in between. At the end paint it with matching colour. It is a longish process. Sandpaper would be 440 grid, no more, no less. No wet sanding.